1. Field of the Invention
The present invention relates to an actuator having an asymmetric rigid structure and method for driving the same. More particularly, the present invention relates to an actuator having an asymmetric rigid structure for changing an optical beam path and a method for driving the same, in which a path of an incident beam can be adjusted in a simple manner and an inclination angle of a reflection beam with respect to the incident beam can be varied from an acute angle to an obtuse angle.
2. Description of the Prior Art
Generally, an incident beam emitted from a beam source is reflected by a mirror and is projected onto a screen such that the proportion of breadth to length is in the ratio of 16 to 9.
In a conventional actuator for driving a reflecting plate which reflects the incident beam, as shown in FIG. 1, a projection lens 4 is disposed in a case (not shown) in order to project the incident beam emitted from an image projection section towards a screen 2.
The image projection section comprises a beam source 10 disposed in a cover 22 so as to emit color beams such as red, green, and blue, a lens 12 disposed in the front of beam source 10 for focusing the color beams emitted from beam source 10, and a dichromatic mirror 14 for reflecting an incident beam introduced through lens 12.
A projection lens 4, which is inclined with respect to dichromatic mirror 14, is installed in a position to receive light from dichromatic mirror 14. An inclination angle and a movement of dichromatic mirror 14 are adjusted by an actuator 20, shown in FIG. 2a, in such a manner that the incident beam introduced through lens 12 can be projected onto screen 2 along a predetermined incident path.
Actuator 20 comprises a support plate 24 installed in a cover 22, a cantilever bar 26 having a fixing end connected to support plate 24 and a free end, and a piezoelectric element 28 attached to a center of an upper surface of bar 26 so as to generate a piezoelectric phenomena. Dichromatic mirror 14 is disposed on the free end of bar 26.
In the conventional actuator having the above construction, the incident color beams of red, green and blue, which are sequentially emitted from beam source 10, are projected onto dichromatic mirror 14 through lens 12. Then, dichromatic mirror 14 is driven by actuator 20 so as to allow the incident beams to have a predetermined inclination angle and a predetermined incident path. After that, dichromatic mirror 14 reflects the incident beams onto screen 2.
That is, when a voltage is applied to piezoelectric element 28, the free end of bar 26 is subjected to a bending stress as shown in FIG. 2b, thereby adjusting the inclination degree of dichromatic mirror 14 according to the incident beams introduced into dichromatic mirror 14.
Accordingly, the inclination degree of dichromatic mirror 14 is adjusted according to a difference of electric potential applied to piezoelectric element 28. Therefore, the incident path of the incident beam and the inclination angle of the reflection beam with respect to the incident beam are changed according to the inclination degree of dichromatic mirror 14. The reflection beam is projected onto screen 2, thereby creating a color image having a predetermined length and breadth ratio.
However, because the incident path of the incident beam and the inclination angle of the reflection beam with respect to the incident beam are changed by the bar which is bent by the difference of electric potential applied to the piezoelectric element, the bar is subjected to a hysteresis when the actuator is used for a long time. For this reason, the path of the incident beam and the inclination angle of the reflection beam with respect to the incident beam cannot be precisely adjusted, and accordingly, it is difficult to precisely project the reflection beam onto the screen.
In order to solve the above problem, another conventional actuator 40 for changing a beam path has been suggested. As shown in FIG. 3, actuator 40 comprises a pair of support plates 42 which are fixed in cover 22 forming the image projection section and are spaced a predetermined distance apart from each other. Support plates 42 are attached to a base 41. A dichromatic mirror 44 for reflecting the incident beam introduced thereto through lens 12 from beam source 10, is disposed between support plates 42. A metal plate 46, on which dichromatic mirror 44 is attached, has a first end of each torsion bar 48 fixed to a respective side thereof. The second end of each torsion bar 48 is hinged to a respective support plate 42.
Disposed below metal plate 46 are first and second driving electrodes 50 and 52 which define an axis that is perpendicular to that of the torsion bars 48. Driving electrodes 50 and 52 are disposed on base 41 in such a manner that the reflection beam can be emitted into lens 4 by adjusting the inclination angle of the reflection beam and the incident path of the incident beam according to the position that the incident beam is introduced into dichromatic mirror 44.
In the actuator for changing the beam path having the above construction, when the incident beam emitted from beam source 10 is emitted into dichromatic mirror 44 through lens 12, dichromatic mirror 44 permits the incident beam to have a predetermined inclination angle and a predetermined incident path, and guides the incident beam onto screen 2 by reflecting the incident beam.
When it is required to change the inclination angle and the incident path of the reflection beam projected onto screen 2, a voltage is applied to at least one of first and second driving electrodes 50 and 52, for example to the first driving electrode 50. Then, as shown in FIG. 4a, metal plate 46 is inclined towards first driving electrode 50 by a static electricity attraction force which is generated by an electrostatic induction due to the difference of electric potential applied between metal plate 46 and first driving electrode 50.
Accordingly, the incident beam is reflected by inclined dichromatic mirror 44 and is projected onto screen 2 while having the predetermined incident path and inclination angle (acute angle). At this time, a torsion is still applied to torsion bars 48 which are fixed to both sides of metal plate 46.
In addition, when it is required to return dichromatic mirror 44 to its initial position or when it is required to make an obtuse inclination angle by inclining metal plate 46 in the reverse direction as shown in FIG. 4b, the voltage applied to first driving electrode 50 is turned off such that the difference of electric potential between first driving electrode 50 and metal plate 46 is removed. Then, if the difference of electric potential is applied between second driving electrode 52 and metal plate 46, metal plate 46 is returned to its initial position by a restoring force of torsion bars 48 and by the voltage applied to second driving electrode 52. At the same time, a static electricity attraction force acts between metal plate 46 and electrode 52 so that dichromatic mirror 44 attached to metal plate 46 is inclined in the reverse direction, thereby making an obtuse inclination angle of the reflection beam with respect to the incident beam.
However, since the actuator changes the incident path of the incident beam and the inclination angle of the reflection beam with respect to the incident beam by static electricity, the metal plate on which the dichromatic mirror is attached adheres to the first driving electrode or the second driving electrode and is not easily separated therefrom. In addition, since the torsion bars are subjected to torsion force when the metal plate is inclined in one direction, the torsion bars are easily fractured as compared with bars that are subjected to bending stress. Moreover, the actuator requires first and second driving electrodes, thus electric power is unnecessarily consumed and the conduit structure is complicated.